Method of bonding dissimilar metals



Feb. 27, 1940.

c. aswAm z ET AL METHOD OF BONDING DISSIMILAR METALS Filed Nov. 5, 1938Leadi Silk/er I @J &

y mm m '10 or coating Patented" Feb. 27, 1940 UNITED STATES P TENTOFFICE 2,191,598 METHOD or nonmnc mssminlm METALS Carl E. Swartz,Cleveland Heights, and Elmore J.

Dockstetter, East Cleveland, Ohio, asslgnors to The Cleveland GraphiteBronze Company, Cleveland, Ohio, a corporation of Ohio ApplicationNovember 5, 1938, Serial No. 239,115

4 Claims.

The present invention relates to a method of bonding or permanentlyuniting two dissimilar metals which normally are of such characteristicsand, properties as to have relatively low aflinity I one for the other,and are therefore essentially immiscible in either liquid or solidstates and do not form chemical compounds or alloys, commonly known asintermetallic phases. More particularly, the invention relates to thebonding of non-ferrous metals such as lead or silver to a ferrous metalsuch as iron or steel.

We have found that such dissimilar-metals can be bonded or permanentlyjoined one to the other by the provision of certain operativeconditions, and without requiring the presence of an additional bondingor fiuxing agent such as solder, tin or copper plate or the like. Suchoperative conditions, as embodied in our present method, also eliminatecertain prior steps heretofore generally found necessary, in the bondingor coating of one metal with another, such as scrubbing, cleaning,pickling and rinsing. In our co-pending' applications Serial No. 91,730,filed July 21, 1936, and Serial No. 195,986, filed March 15, 1938, andof which the present one is a continuation in part, it is indicated thatlead or silver can be directly bonded to steel by the employment oftemperatures substantially above that of the melting point of lead.

The present invention is based upon the discoveries (1) that actualbonding begins only at an elevated temperature with respect to themelting'point of the lead and at a temperature which approximates thatof the gamma iron formation temperature of the iron or steel to whichthe lead is to be bonded, (2) that the bonding becomes easier to effectas the temperature is further increased up to at least 2400 F., and (3)that actual bonding only occurs between such a metal as lead andchemically clean steel of low and medium carbon content, say 50%, suchas is used for boiler plate, deep drawing steel and the like andstructural steels and the like in the form of sheets, strips, rods andwire, that is such steels in finished or semi-finished form ready forfabrication or for further finishing operations. We have also found thatthe utilization of such a bonding temperature range and other conditionsis applicable not only to the bonding of lead and steel, but also to thebonding of a ferrous metal with silver.

To the accomplishment of the foregoing and related ends, said invention,then, consists'of the steps hereinafter fully described and particulli'larly pointed out in the claims; the annexed (Cl. 91-703) I drawing andthe following description setting forth in detail one approved method ofcarrying out the invention, such disclosed method, however, constitutingbut one of the various ways in which the principle of the invention maybe used. 5

In. said annexed drawing: i

Fig. 1 is a more or less diagrammatic view illustrating one form ofapparatus for practicing the process of our invention; Fig. 2 is aphoto-micrograph vof steel coated with lead according to our l0 presentprocess; and Fig. 3 is a photo-micrograph of steel coated with or bondedto silver according to our process.

In the practice of our process, the ferrous metal, such as steel oriron, is introduced into a 16 controlled atmosphere heating furnace. Thefer rous metal may be in any form, unfinished, finished or semi-finishedor even fabricated condition, such as a'sheet, wire, rod, bar, .wovenwire screen or the like. In Fig. 1 of the drawing this 20 step isillustrated by way of the continuous steel strip I led into the heatingfurnace 2. The atmosphere in this furnace is determined by theintroduction of gas, such as commercial annealing hydrogen, or partiallyburnt natural or arti ficial gases produced by commercial controlledatmosphere units, into the inlet 3. The temperature and concentration ofthe gases in the inte-' rior ofthe heating furnace 2 are such as toreduce the oxides in the surface of the steel strip l and to place it inachemically clean condition so that there will be no foreign orobstructing substance tending to interfere with the proper contact ofthe subsequently applied non-ferrous metal.

The following chemical equation illustrates the nature of thedeoxidizing reactions taking place in the heating furnace 2:

In order to prevent the access of air to the surface of the steel stripI, the lower end 5 of the heating furnace, which may be in the form of aquartz tube, is led under the surface of the nonferrous metal bath 6.The temperature of the non-ferrous metal bath is maintained at anytemperature between its melting point and the temperature of the steelstrip. We have discovered that by maintaining the steel strip at itsgamma iron transformation range or above, that 50 it is possible tosecure a direct bond between the non-ferrous metal and the steel withoutnecessity of applying a flux to the latter. Reference to the iron carbondiagram will show that this gamma iron transformation temperature 6grain boundaries of the steel; in other words,

range begins at 133"! for complete transformation, depending upon thecarbon content of the steel or iron. Therefore, the temperature of thesteel strip I emerging from the heating furnace 2 should at or'above itsgamma iron transformation t mperature range at the time it is introducedinto the non-ferrous metal bath 6. -We have found that thehigher thetemperature the faster and more firm will be the non-ferrous metal bondwith the ferrous metal.

After the steel strip I, with its coating of nonferrous metal is led outof the non-ferrous metal bath 6, it may be quenched by means of thewater spray 1. Any suitable cooling means may be employed since'the bondis determined and affected, not by the cooling but by the temperature ofthe two metals and bythe condition and carbon content of the ferrousmetal.

A different type of apparatus may be employed for the performance of ourprocess, as long as the required conditions of operation are maintained.Thus a centrifugal casting apparatus such as shown and described in ourco-pending application Serial No. 91,730, filed July 21, 1936 or U. S.Patent No. 1,923,075, may be employed, with appropriate change inconditions of temperature and atmosphere as hereinabove described.

Where lead is the non-ferrous metal employed in the bath 6, a bond willresult as illustrated in the photo-micrograph of Fig. 2. Similarly,where a silver bath is employed, the bond as shown in thephoto-micrograph of Fig. Both of these photo-micrographs illustrate theunusual irregularity of the bond line, thus showing that there is anextremely effective physical adhesion between the two dissimilar metals.The photomicrographs also illustrate the preferential effect which thenon-ferrous metals have for the the non-ferrous metal has penetratedinto the surface of the steel, and in some areas, it will be noted thatin the case. of lead (Fig. 2) that the bonding line is so irregular thatthere are in effect peninsulas of steel surrounded by lead, and that inthe case of silver (Fig. 3) the ferrite grains are completely isolatedin the silver layer. This difference as to the amount of penetration oflead as compared to silver into the steel may 1 be possibly explained bythe fact that the solubility of iron in lead is not quite as great asthe solubiliy of iron in silver, although for other than theoreticalpurposes, the solubilities are nil. Thus, it has been determined thatthe solubility of of iron in lead (at elevated temperatures) is in theorder of 2 to i -times 10- per cent, and that the solubility of iron.insilver is 4 to, 6 times 10- per cent.

This preferential j-efiect of the non-ferrous metal along the grain'boundaries of the steel apparently accounts for the irregular bond line.The possible use of a decarburizing atmosphere in the treatment of thesteel before it is immersed in the non-ferrous metal bath will enhancethe production roughened and irregular surface.

A possible explanatory, factor pointing to the of the direct bondbetween the steel as lead and sileflect on the steel and the non-ferrousmetal such F. and proceeds to 1663 F. I

process herein 3 results.

ver, and at the critical temperature range specifled, is to be found inthe fact that the precipitation of ferrite from the solid solution ofaustenite occurs in this temperature range. Thus, the line ordinarilyreferred to in the iron carbon diagram as A3 is sometimes called theferrite solubility line. Therefore, as steel is heated to a range atorabove this ferrite solubility line (vim, 1337? to 1663 F.) the ferritebegins to enter solution. Since, as shown in Figs. 2 and 3, thenon-ferrous metals such as lead and silver have a preference for thegrain boundaries rather than the grains of ferrite themselves, it can beconcluded that the absence of ferrite, or its entering into solution,enhances the bonding or adhesion effect of the non-ferrous metal to thesteel.

Other modes of applying the principle of our invention may explained,change being made as regards the steps stated by any of the followingclaims or the equivalent of such stated step or steps be employed.

We therefore particularly point out and-distinctly claim as ourinvention:

1. The method of coating ferrous metal sheets, strips and the likecontaining not more than .50% carbon with lead comprising the steps ofheating the ferrous metal at or above its gamma iron transformationtemperature range and in an atto produce a chemically clean mospheresuflicient surface thereon, and then, while the metal is at or abovesaid temperaturerangaand before the metal has been exposed to the air,conducting it into and through a coating bath of molten lead.

2.. The method of coating ferrous metal sheets, strips and the likecontaining not more ,than 50% carbon with lead, comprising the steps ofheating the ferrous metal to a temperature not less than that of thegamma iron transformation tempera? ture range and not exceeding 2400" F.and in an atmosphere sufiicient to produce a chemically clean surfacethereon, and then, is at such temperature and before it has been exposedto the air, conducting it into and through a coating bath of moltenlead.

3. The method of lead coating ferrous metal sheets, strips and the like,containing not more than 50% carbon, comprising the stepsof heating theferrous metal to a temperature not less than about 1337" F. and not morethan about 2400 F. and in an atmosphere sufiicient to deoxidize thesurface thereof, and then, metal is at such temperature and beforeexposing the metal to oxidizing influences, conducting it into andthrough a coating bath of molten lead.

4. The method of coating ferrous metal sheets, strips and the likecontaining not more than .50% carbon with lead comprising the steps ofheating the ferrous metal at or above its gamma iron transformationtemperature range and in an atmosphere sufiicient to produce achemically clean surface thereon, and then, while the metal is at orabove said temperature range,-and before the metal has been exposed tothe air, applying a coating of molten lead thereto.

CARL E. SWAR'IZ. EL'MORE J.

be employed instead of the one disclosed, provided the step orwhile thewhile the metal

